Ultra High Molecular Weight Polyethylene (UHMWPE) is a commonly used articulation material, and wear of this material is considered a major factor that contributes to the aseptic loosening that limits the duration of artificial joints. Reducing UHMWPE wear has been an important topic of research and development since the sixties, and investigators have worked with increasing urgency on this problem for about the last decade.
Since its introduction into total joint replacement by Charnley, UHMWPE remains the most commonly used material for the concave part of artificial joints (e.g., for the acetabular cup of the total hip prosthesis, or the tibial plateau of the total knee prosthesis). While it has played a central role in the success and widespread use of joint replacements, UHMWPE has also been identified as a major culprit in the most common mode of their failure—the aseptic loosening mentioned above. Wear particles produced by articulation of the hard, convex, usually metallic, or ceramic component against the soft polymer liner accumulate in and around the joint until the concentration of particles becomes so high that, in spite of the polymer's generally excellent biocompatibility they initiate a biological response leading ultimately to bone loss, loosening of the joint components, and dysfunction of the joint replacement.
Different methods of wear reduction have been sought, found, and introduced into clinical use over the last few decades. These methods were focused on improving the wear characteristics of the articulation pair at their interface.
When directed to the convex, metallic, or ceramic component, these wear-reduction efforts aimed to reduce roughness of the surface, increase the precision of the geometry (e.g., by improving the sphericity of the femoral head), and increase the hardness of the materials. Improved wetting of the hard surface has also been identified as an important factor in general wear reduction.
These measures have been realized through: (i) better selection and processing of metals, both in metallurgical aspects and in the machining methods employed; (ii) use of hard coatings, which can be added or created in situ (e.g., by oxidation); and (iii) use of bulk ceramic components.
The reduction of wear, both in laboratory testing and based on in vivo observations, is on the order of about a factor of two.
More recently, modifying UHMWPE material by cross-linking has attracted attention. Cross-linking can be achieved by physical means (e.g., irradiation) or chemical means. The results of laboratory testing have varied in part due to the different methods used to produce wear and the methods of assessing the resulting wear. Tests conducted on joint simulators and with careful compensation for artifacts, suggest a five- to ten-fold reduction of wear when cross-linked UHMWPE, is compared to “regular” UHMWPE.
However, there are a number of potential problems with cross-linking. These include a reduction of strength, particularly in fatigue; a reduction of average particle size, which makes the wear debris more biologically active; and risk of long term degradation in the body.
Some would argue that cross-linked UHMWPE has not been in broad clinical use for long enough to reach a conclusion on its benefit-to-risk ratio. Several clinical observations suggest the actual wear reduction has been reduced by about a factor of two but, again, there is much variability in methods used to assess the wear and thus in the reported results.
Metal-metal articulations were used before UHMWPE was introduced. As the biological problems with wear of UHMWPE surfaced in the eighties, metal-metal articulations were re-introduced, and those supported by better metallurgical and manufacturing technologies consequently promised better clinical outcomes. The wear rates, compared to UHMWPE, are lower by about a factor of ten by weight and by about a hundred times by volume. However, systemic accumulation of ions of potentially harmful metals has been observed, and the risks of this accumulation remains unknown, especially in younger patients, who are most in need of improved joint replacements.
Ceramic-ceramic articulations are technically the best in terms of wear, but various regulatory obstacles and high prices have, until recently, limited their use. There is also a risk, albeit low, that ceramic components, which are brittle, can break. Any damage to a surface or miniscule imperfections can cause rapid degradation of the articulation. Sophisticated technologies and quality controls required in production have also been an impediment to the wider use of ceramics.
Swiss patent CH449173, entitled “Gelenkprothese,” by Maurice Mueller, discloses a metal on metal prosthesis, whereby the contact is limited to polymeric pads seated into recesses within the cup.
German patent DE4423020, entitled “Gelenkprothese,” by Wolfgang Fitz, discloses a hip prosthesis cup with a reservoir for lubricating fluid in the unloaded, inferior region, combined with grooves, which are known in the general art of sliding bearings as advantageous because wear particles are more readily removed from the articulation.
German patent DE19604458, entitled “Gelenkpfanne,” by Hagen Seifert, discloses a hip prosthesis having a cup shaped so as to make exclusively a ring contact to the head near the equator, leaving a spherically shaped recess in the cup filled with fluid. The fluid-filled recess is enabled by grooves in the contact area to act as a shock absorber. To control the stiffness of the cup, there are a number of ring-shaped cavities within the cup wall. The concept is essentially one of a hydrodynamic bearing. Devoid of fluid support, as would happen in any case because the loading must persist for just fractions of a second, the prosthesis would either experience very high friction torque (where the load is supported by the near-equator contact zone) or it would be reduced to a standard bearing (plus the friction at the contact zone) if the head fell into the recess.
German patent DE19915814, entitled “Gelenk-Endoprothese mit verschleissarmer Gleitpaarung,” by Manek Buttermilch et al., discloses a ceramic-ceramic total hip prosthesis in which the contact between the two articulating components is a line contact, the line formed by either a modified head geometry or modified cup geometry. In either case, the mismatch is produced by replacing a single radius of curvature by two; the first radius and the second radius have offset centers, resulting in a line contact. Herzian stresses are reduced but not eliminated. The geometry of this invention is also characterized by the fact that the aspherical component of the articulation presents a kink (the two circles defining the cross-section of the aspherical component are not tangent) in its contour at the line of contact.
European patent EP0053794, entitled “Cup for a hip joint,” by Manfred Semlitsch, et al., discloses an endoprosthesis in which both the joint ball and the hip joint cup consist of oxide ceramic material. An annular recess is arranged in the area of the opening of the hip joint cup, and a ring of bioinert, plastically deformable material is situated in the recess. The surface of the ring that faces the joint ball merges essentially without interruption and entirely steplessly into the spherical surface of the cup. In the event of subluxation and an associated short-term, linear-type bearing contact between joint ball and joint cup in the edge area of the cup, the result, even in the case of dry friction, is a favourable tribology between the joint ball and the ring of plastically deformable material which comes into engagement with the ball upon subluxation.
European patent EP0821922, entitled “Hip prosthesis joint component with particulate trap,” by Claude Hubin and Marie Jean Sterpin, discloses a hip prosthesis cup for metal-metal articulation provided with a polar recess that serves as a trap for wear particles. Alternatively, the head can also have a trap or recess.
French patent FR2727856, entitled “Ensemble prothetique auto-lubrifiant pour Particulation de Ia hanche,” by Barba Laurent et al., discloses a hard-hard (metal-metal, or ceramic-ceramic) articulation for a total hip prosthesis. The shape and dimensions are such that a laminar film of synovial fluid can be maintained in use. A reservoir for the fluid is provided at the polar region. No details are provided as to what the geometry of the cup should be to meet the requirement of fluid film lubrication. There is a teaching that the gap between the cup and the head should be in the range of 0.005 to 0.05 mm. This covers the standard radial clearance used in hard-hard bearings.
Great Britain patent GB1322680, entitled “Improvement in and related to prosthesis,” by Georges Girard and Ramiro Cameo, discloses a total hip prosthesis in which the surface of the head is provided by a pattern of grooves intended to reduce the wear at the articulation. As prior art, the inventors cite a prosthesis in which the spherical head articulates against a cup that is “football shaped.” That is, the cup is elongated, which leads to a line contact, rather than a point contact.
US patent application US2002/0116068, entitled “Containment system for constraining a prosthetic component,” by Terry McLean, describes a truncated head within a total hip prosthesis that can be inserted into the cup sideways through slots in the opening of the cup before it is turned into a functional position. As a result, the head is retained within the cup, which covers more than 180 degrees. The unintended result is that the conventional point contact is now changed into a line contact along the edge of the truncated segment of the head.
US patent application US2005/0246026, entitled “Modular orthopedic implant apparatus,” by Paul Lewis et al., discloses a modular acetabular cup comprising three elements, that can be combined in different ways to allow the surgeon a choice of implants of varying sizes and features. Fixation is achieved through a central bore through all three components. As in U.S. Pat. No. 6,527,809, one can change the point contact into line contact along the edge of the liner.
US patent application US2005/0261776, entitled “Prosthetic joint with annular contact bearing surface,” by Scott Taylor, discloses a truncated, or annular, acetabular component of a total hip prosthesis in which contact between the head and the inner of two members of the cup occurs along a line instead of at a point.
U.S. Pat. No. 5,181,926, entitled “Bone implant having relatively slidable members,” by Rudolf Koch and Robert Streicher, describes a total hip prosthesis in which the cup side, within cavities in its polymeric liner, contains self-aligning pads of hard material that articulate against the head.
U.S. Pat. No. 5,549,693, entitled “Cotyloidal prosthesis,” by Christiane Roux and Michel Pequignot, discloses a total joint prosthesis in which the cup side contains at its opening a ceramic ring, much like a natural labrum, which forms a seal with the ceramic head. The position of the ring is such that the frictional moment of the couple would be very high.
U.S. Pat. No. 5,593,445, entitled “Bi-axial prosthetic joint,” by Thomas Waits, describes a total joint prosthesis in which a third, ring-shaped, member is interposed between the head and the cup. The ring-shaped member increases the contact area under load, between the head and cup self-aligning between the head and the cup in the direction of the load.
U.S. Pat. No. 5,702,456, entitled “Implant having reduced generation of wear particulates,” by David Pienkowski, discloses a method of pre-wearing the prosthesis before implantation so the particles usually produced in somewhat higher amounts by the wear-in process do not burden the body. Only a minimal improvement in long term outcome could be expected from such a procedure.
U.S. Pat. No. 5,725,593, entitled “Total anatomic hip prosthesis,” by Francesco Caracciolo, discloses a resurfacing total hip prosthesis. The femoral cup has multiple circular rises that are intended to reduce the friction within the spherical cup.
U.S. Pat. No. 5,766,258, entitled “Wrist prosthesis,” by Beat Simmen, discloses a wrist prosthesis in which, in one embodiment, one of two separate articulations is produced with non-circular members. The members tend to fall, or self-center, into a stable position in which they become congruent.
U.S. Pat. No. 6,527,809, entitled “Trial acetabulum or implantable acetabulum with adjustable orientation,” by Levon Doursounian and Michel Porte, discloses a modular acetabular cup. The cup inlay, which articulates against the head, has a central opening allowing access to a mechanism that locks the cup in a desired position. This, as a side effect, defines the contact conditions between the head and the inlay as a line contact along the edge of the central opening, as is the case in U.S. Pat. No. 4,840,631. “Artificial hip joint socket with hydraulic head support,” by Robert Mathys. However, the modular prosthesis described by Doursounian and Porte lacks the hydraulic pressure support disclosed by Mathys.
U.S. Pat. No. 4,031,570, entitled “Prosthetic acetabutum,” by Otto Frey, discloses a torus-shaped aspherical cup in which the radius of the curvature is equal to that of the spherical head, but the center of the curvature is offset from the central axis so as to avoid jamming of the head in the cup. This changes the theoretical point contact of a sphere in a spherical socket into a line contact of a sphere in a toroidal socket, and further, for the purpose of improved lubrication, a groove at the periphery of the cup and a recess/pocket at the pole. Herzian stresses are reduced by changing from the point to line contact, but they are not eliminated by the current invention which changes point contact to surface contact.
U.S. Pat. No. 4,840,631, entitled “Artificial hip joint socket with hydraulic head support,” by Robert Mathys, discloses a hip joint articulation with a cylindrical recess machined into the cup. This recess creates a reservoir for the joint fluid, which would pressurize under load. Sealing is provided by the edge of the recess. The disadvantage of this arrangement is that high stresses are produced at the edge of the recess, which could lead to localized wear, and potentially to loss of the seal and hence of hydraulic support.
U.S. Pat. No. 5,336,267, U.S. Pat. No. 5,383,936, U.S. Pat. No. 5,738,686 and U.S. Pat. No. 6,312,471, by Dietmar Kubein-Meesenburg et al., disclose the theoretical basis for and solutions to reducing stresses in articulations of joint prosthesis, all of which lead to theoretical line contact instead of point contact. Herzian stresses are reduced, but not eliminated.
GB Patent No. 1322680, entitled “Prosthesis,” by Georges Girard et al. discloses a metal-metal total hip joint articulation. The prosthesis includes a concave cup having multiple grooves leaving only protrusions ending on a spherical surface to contact the spherical head. This type of contact is proposed to reduce the risk of jamming of conventional, smooth, spherical surfaces of a ball-in-socket joint, specifically in metal-metal combination, where the required tolerances are tight and difficult to maintain in production.
U.S. Pat. No. 6,645,251, entitled “Surfaces and processes for wear reducing in orthopedic implants,” by Abraham Salehi et al. discloses an approach based on grooving the concave surface in order to improve lubrication and distribute the stress. As is known from technical sliding bearings, the main advantage of grooves comes from improved removal of wear particles away from the articulation. Fluid entrapment may play a role in improved lubrication as well. However, grooves as disclosed may in fact lead to higher local stresses at the edges of the grooves and defeat the purpose. There has been no published data supporting the concept and no evidence of acceptance of this approach by the orthopedic device industry.
U.S. Pat. No. 6,425,921, entitled “Sliding partners for artificial joint implants,” by Hans Grundei and Wolfram Thomas, discloses an alternative approach where grooves are produced in the convex component of the joint. Actual hip simulator tests performed on this type of joint components did not show any wear reduction.
Practical limitations on the accuracy of the machinable components, including the effects of temperature and of radiation-induced shrinkage (which may be used for sterilizing polymeric cups) and the need for safety precautions (e.g., against jamming of the components when used in the body) have led to international standards which help to guarantee acceptable in viva performance.
ISO standards 7206-2; 27.80 to 28.00 and 7206-2; 28.10 to 28.30 specify the geometry and dimensions of the head and cup components, respectively, of a total hip prosthesis. For example, regarding the sphericity and dimensional tolerance of the head component, the standard specify that the metal or ceramic femoral head component of a total hip prosthesis shall have a departure from roundness of not greater than 10 micrometers. If used against hard material cups (metallic or ceramic) it will not be greater than 5 micrometers. The diameter shall be equal to nominal diameter +0.0, −0.2 mm. For metal-metal or ceramic-ceramic articulations the tolerances are not specified, but in all cases there shall be radial clearance. In practice, the heads in use today are produced with significantly tighter specifications than required by the standards.
Regarding sphericity and dimensional tolerance of the cup component, for polymeric components, the sphericity is not specified. For hard materials, it shall not exceed 5 micrometers. The dimensional tolerance for a polymeric cup is +0.3, +0.1 mm at 20±2° C. from the nominal diameter. In practice, UHMWPE cups are oversized by at least +0.2 mm over the nominal diameter. No tolerances for the metallic or ceramic cups are given, but the radial clearance must be guaranteed by the producer. Typical radial clearance for hard pairs is in the range of 0.02 to 0.03 mm.